EFFECT OF SOIL FACTORS OX WATER REQUIREMENT. < 
The results of this experiment show that when rye follows potatoes 
the yield and water requirement are not measurably different from 
that obtained when rye follows rye. When rye follows barley, how- 
ever, the yield is increased and the water requirement is reduced, com- 
pared with rye following rye. The lower water requirement would 
naturally result from the increased supply of plant food hi the barley 
pots, as indicated by the higher yields. 
EFFECT OF SOIL TEMPERATURE OX THE WATER REQUIREMENT. 
king's experiments. 
King (1895) compared the water requirement of potatoes when 
grown in pots above ground with similar determinations made hi pots 
buried hi the soil. The pots above ground were protected from the 
direct rays of the sun by a board screen placed on the south side. 
Galvanized-iron pots 18 inches in diameter and 24 niches high were 
used. The plants were covered at night and during rams with a tent 
of heavy duck. It was found that the water requirement was appar- 
ently less wnen the pots were placed hi the .ground, but the difference 
observed can not be considered conclusive, since it is less than three 
times the probable error (Table XL VI). Xo data are given regarding 
the temperature fluctuations ia the two series of pots. It seems 
probable that the more exposed position of the plants grown hi the 
pots above ground would also affect the results. 
Table XLVI. — Effect of soil temperature on the water requirement of potatoes. 1 according 
to King {1895, p. U 
Treats o, pot, ^ Water. ^ST 
Water re- 
luiremeni 
based on 
drv matter. 
Grams. Kilograms 
227. 7 
Above ground { 1G3. 
m i 
Sunk in potato field { |j|- £ 
107.4 
97.4 
112.9 
472 
J597 
458 
509±3o 
100.1 
99.0 
430 
415 
423±6 
1 Alexander's Prolific. * King gives 497, which is evidently an error. 
leather's experiment^ 
The pot-culture method of measuring the water requirement of 
plants subjects the soil to greater temperature fluctuations than occur 
under field conditions, owing to the exposure of the pots to the direct 
rays of the sun. To determine the effect of this exposure, Leather 
(1910) thermally insulated jars in which maize and wheat were grow- 
ing by surrounding them with from 5 to 7 niches of sawdust through- 
out the period of growth. These were compared with jars fully 
285 
